This invention relates to a vacuum-type circuit breaker comprising a plurality of vacuum circuit interrupters, motive means for developing a closing force for application to the interrupters, and closing linkages respectively connecting the interrupters with the motive means. More particularly, the invention relates to means for substantially reducing the tendency of the movable contacts of the vacuum circuit interrupters to reverse their motion, or bounce away from their respective mating contacts, immediately following contact-engagement near the end of a closing operation while closing force is still being applied to the closing linkages by the motive means.
In my Application Ser. No. 752,956, filed Dec. 20, 1976, now Patent No. 4,099,039 which is incorporated by reference in the present application, I have disclosed how elasticity effects in the closing linkages can produce contact separations of several milliseconds duration shortly after the contacts engage near the end of a closing operation. In the aforesaid application, the solution to that problem involves modifying the effective initial gradient of the pre-loaded wipe springs in the usual wipe mechanisms of the circuit breaker, which mechanisms are used to couple the linkages to their respective interrupters. This approach has been found quite effective in suppressing such contact-separation when the closing linkages are essentially unworm and still have near-minimum clearances in their usual pin joints. But when these clearances increase, due to normal wear during the life of the circuit breaker, and particularly where there are a relatively large number of such pin joints interposed mechanically in series in the linkage between a common operating member and the movable contact rod of an interrupter, some such contact-separation may still occur, albeit of reduced severity and frequency of occurrence.
Vacuum interrupters are particularly susceptible to this contact-separation phenomena because massive butt contacts must be used without recourse to sliding between the mating contacts. As a consequence, only a slight reversal of contact-motion can result in a contact-separation of as much as several milliseconds duration. Such contact-separation is particularly undesirable because it produces severe contact-erosion which limits contact life and also because it increases contact-welding.
My studies have shown that the above-noted contact-separation problem that develops as the closing linkage wears is related to two conditions, the significance of which had not previously been fully appreciated:
(a) the presence in the linkage of a large number of pin joints, mechanically in series, between the common closing operating member and the movable contact rods of certain of the interrupters, and PA1 (b) the natural closing bias force on the movable contact rods of such interrupters which results from the pressure differential across the usual bellows of each such interrupter.
When the contacts of the interrupter are open, the above condition (b) results in the clearances in the pin joints being located on the common-operating-member side of the pin joints. But near the end of the closing operation, when the contacts first touch and wipe-spring force is developed on each contact rod, the loading on the contact rod reverses, causing the pins of the pin joints to shift in their bearings to the opposite side of the bearings. Due to the inherent elasticity of the linkage and to the cumulative effect of the clearances of the several pin joints, it is not possible to transmit a significant closing force to the movable contact rod instantaneously following initial contact-engagement. It is first necessary for the closing force at the driving end of the linkage to shift the pin-to-bearing clearances and then to strain the linkage sufficiently to transmit the required force.
In a conventional linkage, as the bearings wear, the cumulative value of the clearances in the series-connected pin joints becomes quite large; and, as a result, a relatively long time has been required to generate the required force to hold the contacts engaged following initial engagement. During this time the contacts can be temporarily forced out of engagement under the severe loading produced by the wipe springs immediately following initial contact-engagement.